Linear actuators which employ a screw arrangement having a drive screw and a drive nut are used in a variety of applications to transfer a rotational input to a linear output. As one example, a linear actuator may be used in an aircraft to move a control surface, e.g. an aileron, of the aircraft to a desired orientation in order to manipulate flight dynamics.
As is readily understood, a motor or other device provides a rotational input, i.e. an input torque, to rotate the drive screw of the linear actuator. A ram of the linear actuator is in threaded engagement with the drive screw via the drive nut. The ram is mechanically constrained from rotating with the drive screw by the use of a keyway or similar configuration. As a result, as the drive screw rotates, the ram moves linearly as a result of the aforementioned constraint. This linear output of the actuator is used to adjust a position of the control surface in this exemplary application.
Due to the employment of a screw arrangement, the linear actuator is highly efficient in holding the desired position of the control surface with minimal, if any, input holding power. Indeed, once in a desired position, the control surface will hold its orientation with minimal to no input holding power because it is very difficult to release or disengage a linear actuator which employs a screw arrangement. This capability is due in large part to the low lead angle of the threads of the drive screw and the drive nut.
Unfortunately, there may be instances where it is desirable to release or disengage the linear actuator. For example, and still referring to the example of a control surface above, if the input drive device, such as a motor, connected to the linear actuator fails and cannot return the control surface to a neutral position, the control surface must be biased back to its neutral position by an external input. This biasing operation requires releasing or disengaging the linear actuator. However, as stated above, it is very difficult to release or disengage a linear actuator which employs a screw. In such circumstances, the control surface thus becomes jammed in a certain orientation.
There have been attempts to construct a linear actuator which employs a screw arrangement that can be released or disengaged. Such systems employ various mechanisms for selectively disengaging the ram from the drive screw so that the ram may be freely moved without the resistance created by the screw. See for example U.S. Pat. No. 3,669,440 to Kartasuk et al. titled Quick Engaging and Disengaging Nut Mechanism; U.S. Pat. No. 6,142,447 to Jean-Luc et al. titled Translation Block with Disengaging Nut for Screw Mounted Lifting Mechanism; and U.S. Pat. No. 5,101,679 to Smith et al. titled Screw Drive Engagement/Disengagement and Decoupling Mechanism, the teachings and disclosures of each of which are incorporated by reference herein in their entireties.
However, as can be seen from the aforementioned patents, the mechanisms required to permit such back driving are relatively complex and have a high part count. Further, such designs result in a non-symmetrical loading on the drive screw by the mechanisms which are used to selectively disengage the ram from the screw.
Accordingly, there is a need in the art for a linear actuator which, while employing a screw arrangement, may still be released or disengaged as needed without relying on a complex disengaging mechanism and without non-symmetrical loading on the drive screw. The invention provides such a linear actuator. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.